1. Field of the Invention
The invention relates to aircraft gas turbine engines and, particularly, to transition ducts between low pressure and high pressure turbines of such engines.
2. Description of Related Art
A gas turbine engine of the turbofan type generally includes a forward fan and a booster or low pressure compressor, a middle core engine, and a low pressure turbine which powers the fan and booster or low pressure compressor. The core engine includes a high pressure compressor, a combustor and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor. High pressure air from the high pressure compressor is mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Most of the thrust produced is generated by the fan. Marine or industrial gas turbine engines have low pressure turbines which powers generators, ship propellers, pumps and other devices while turboprops engines use low pressure turbines to power propellers usually through a gearbox.
Large modern commercial turbofan engines have higher operating efficiencies with higher by pass ratio configurations and large transition ducts between the low pressure and high pressure turbines. In order to improve efficiency of the low pressure turbine, a gas flowpath in a transition duct between the high and low pressure turbines is expanded to a greater radius. It is greatly desirable to have a transition duct with a shortest length as possible in order to save engine weight. However, if the axial length of the transition duct is too short it becomes a rapidly expanded duct that causes the flow near the boundary layer to separate from a radially outer wall of the transition duct and cause undesirable pressure losses. It is desirable to have a design that can prevent the flow separation and still have a rapidly expanded and short transition duct.